The present invention relates generally to fiber optic communications systems and methods, and more particularly, to the use of band efficient modulation formats to increase data transmission rates for a given nonlinear distortion limited fiber link, or increase the distortion-limited distance for an optical fiber link at a given data rate.
The deleterious effects of nonlinear distortion in combination with chromatic dispersion slope in optical fiber transmission increase with symbol rate and fiber length. Nonlinear distortion means that the refractive index of the fiber varies according to the instantaneous amplitude of the light being transmitted. Therefore, the instantaneous propagation velocity of the light varies according to its instantaneous amplitude. This causes the instantaneous phase of the light to be perturbed, which causes the optical spectrum of the symbol to broaden. This broader optical spectrum is much more subject to chromatic dispersion, which distorts the symbol in amplitude. This distortion ultimately limits the maximum symbol rate that can be transmitted over a given length of fiber.
Previous solutions that attempt to mitigate against this include using expensive optical equalization techniques and return to zero data formats versus non-return to zero. Note that these solutions and others universally are based on simple On-Off-Keying (OOK) of the optical carrier.
With the prior art OOK modulation format, symbols are affected dissimilarly, because the amplitude versus time in each symbol differs. For example, a short pulse xe2x80x9c1xe2x80x9d in the sequence xe2x80x9c010xe2x80x9d will distort differently than the longer pulse xe2x80x9c111xe2x80x9d in the sequence xe2x80x9c01110xe2x80x9d, and so forth. The end result of this situation is an unintelligible mess, with no straightforward means of distortion compensation.
Therefore, it would be advantageous to have systems and methods that use alternative modulation methods that are more robust than OOK versus nonlinear distortion and dispersion slope over a given fiber length, but keep the data rate the same or better. It would also be advantageous to have systems and methods that use phase shift keyed (PSK) modulation to accomplish this.
The present invention provides for systems and methods that use band efficient phase shift keyed (PSK) modulation to increase the data transmission rate through a given nonlinear distortion limited optical fiber link, or equivalently to increase the distortion-limited distance for an optical fiber link at a given data rate. This is achieved by amplitude modulating an optical carrier using a microwave carrier with the data encoded on it through PSK modulation. Using the present invention, distortion-limited data rates and/or distances may be increased without introducing expensive optical distortion mitigating devices.
The principal reason for this improvement is, that with PSK modulation each symbol behaves similarly with respect to optical amplitude dependent distortion. This is because the optical amplitude envelope of each symbol is similar versus time. The only difference is a phase shift of the envelope. Thus the amplitude dependent distortion is the same for each symbol, and although distorted, the symbols stay equally spaced in time. This distortion may be counteracted with predistortion and other equalization techniques on the microwave PSK signal.
The use of phase shift keyed modulation, for example, has the additional advantage that, by increasing the number of bits per symbol and keeping the symbol rate constant, the data rate for a given length of fiber may be increased without introducing additional distortion.